SAN FRANCISCO -- With summer approaching and California's snowpack measuring at a fraction of normal, state water managers are set to order farmers and other big water users to limit the water they take from rivers for the first time since 1977.

The State Water Resources Control Board projected that curtailment letters would be sent out later this month for water users on 10 different rivers and their watersheds, including the Middle Fork Eel -- a major tributary of the Eel River.

While no letters have yet been sent, the prospect is extremely likely, water officials said.

"When the letters do go out, they're effective immediately," said Timothy Moran, a spokesman for the board. "The people who are curtailing have seven days to reply."

The orders will be delivered first to junior water-rights holders -- those who obtained their water rights after 1914 and whose ability to take water is second behind pre-1914 senior rights holders. Senior holders would still be able to take water initially, and only be ordered to curtail if conditions became even more extreme.

The rare measure of ordering curtailments comes amid the third year of withering drought conditions.

Humboldt County has been in an extreme drought since the end of January, National Weather Service hydrologist Reginald Kennedy said.

"As we go into the summer months, it's still somewhat concerning," Kennedy said. "The rangeland grasses will probably dry out earlier than normal. We still have some issues with the rivers running below the normal flow."

On Thursday, state measurements of the Sierra Nevada snowpack found more bare ground than snow.

The Sierra snowpack is an essential element of California's water supply -- it accounts for about one-third of the state's water.

It is now at a mere 18 percent of average for the date.

"Anyone who doesn't think conservation is important should drive up the hill and take a look," Mark Cowin, the California Department of Water Resources director, said in a news release. "Coupled with half our normal rainfall and low reservoir storage, our practically nonexistent snowpack reinforces the message that we need to save every drop we can just to meet basic needs."

The extreme drought conditions have had a ripple effect through the state's environment and economy.

Because of the lack of water, farmers have fallowed tens of thousands of acres and anticipate they won't have work for thousands of farmworkers. Ranchers have had to sell off parts of their herds to cut costs as free-range grasses failed to grow as abundantly as usual.

The lack of water will affect numerous species that live in California's rivers and streams, home to 37 fish species that are endangered.

The state also has responded to twice the number of wildfires this year than usual. It has bolstered firefighting crews to prepare for what is expected to be a busy fire season through the summer and fall.

Gov. Jerry Brown has declared a drought emergency, and on April 25 issued his second drought-related executive order in an effort to help firefighters, farmers and cities more quickly respond to the drought.

The rivers slated for the curtailment orders include the Russian River above Healdsburg and the Kern, Kings, Kaweah, Merced, Middle Fork Eel, Stanislaus, Tule, Tuolumne and Yuba rivers.

Because the board does not have much enforcement staff, its ability to oversee thousands of rights holders to ensure compliance with the curtailment orders will be a challenge. The board will largely rely on reports from neighbors and others to find any violators.